Adsorption of acetylene and hydrogen on Pd(111) : Formation of a well-ordered ethylidyne overlayer

The C2H2(+H)/Pd(111) system has been studied using high resolution core level spectroscopy and low energy electron diffraction (LEED). Two ordered phases were found when adsorbing C2H2 at 125 K, giving rise to a (2 × 2) and a (√3 × √3)R30° LEED pattern, respectively. When preadsorbing H at 110-150 K, the sticking probability of the C2H2 molecules is strongly reduced and only the (√3 × √3)R30° overlayer is formed. In all three of the ordered structures the C2H2 molecules are suggested to occupy the hollow sites. Heating of the (√3 × √3)R30° C2H2 + H layer to 350 K yields a well ordered (√3 × √3)R30° overlayer consisting... (More)